Automatic tuning system



I AMPLITuns E K JL i corIPARIson UNIT "f-`- sept. 26, 195o VAHLE mL2,523,791

AUTOMATIC TUNING SYSTEM UNIT Vig. 4.

InventoI-fs: Julius Vahle, Paul D. Heath,

Their- Attorney SENSITIVE UNIT Sept. 26, 1950 J. VAHLE ET AL AUTOMATICTUNING SYSTEM Filed Nov. 26, 1948 3 Sheets-Sheet 2 o I c 44o` vours JPHASE A-c 22o vous Arf:-

"'IIOVOLTS A.C.

Inventors: Julius Vahle, Paul D Heth,

Tl'weir` Attorney.

Sept. 26, 1950 J. vAHLE ETAL 2,523,791

AUTOMATIC TUNING SYSTEM Filed Nov. 26, 1948 3.Sheets-Sheet 3 +250 V.D.C.

VOLTAGE REGULATED POWER 5U PPLY VOLTAGE REGULATED POWER SUPPLY vInventors: no vous A.c. JLLHLLS Vahle, Paul DHeath,

by m

TheirAttoPney Patented Sept. 26, 1956 AUTOMTIC TUNING SYSTEM JuliusVahle and Paul D. Heath, Syracuse, N. Y.,

assignors to General Electric Company, a corporation of New YorkApplication November 26, 1948, Serial No. 62,173

(Cl. 21S-47) 8 Claims.

Our invention relates to automatic tuning systems for high frequencyelectrical circuits and more particularly to automatic tuning systemsfor circuits associated with the transmissionv and utilization of highfrequency energy for' heating.

It is the object oi our invention automatically to match the impedenceof a heater to the im pedance of the transmission line connecting theheater to a source of high `frequency energy.

It is generally known that the maximum amount of energy can betransferred between two pieces of electrical apparatus when theirimpedances are equal. This applies to ati-ansmission line which suppliesenergy to a high frequency heater. The maximum energy can betransferred, and that energy will be used most eiciently when theimpedance of 'the heater and the network associated `with it, is equalto the impedance of the transmission line. While this is true at allfrequencies, at high frequencies it is particularly important that thetwo impedances be kept in balance. At the same time, it is more diicult,at high frequencies, to keep them in balance. At high frequencies, achange in any of the circuit elements generally has a much more markedeilect on the impedance balance than at low frequencies. An automatictuning system is therefore very important at high frequencies, tocompensate for changes in impedance which occur in the heater and keepthis impedance in balance with the impedance of the transmission line.

In carrying out our invention in one form, we provide a heating chambercomprising two large horizontal parallel electrodes or" electricallyconductive material. The spacing between the two electrodes is manuallyvariable and the material to be heated is passed between them, usuallyin the form of a continuous strip or on a conveyor belt. High frequencyenergy is supplied to the heating electrodes through a concentricconductor transmission line. The automatic tuning system comprises avariable capacitor in series with the electrodes and a variable inductorin parallel with them. The capacitance is Varied automatically tomaintain the phase angle between current and Voltage in the transmissionline at zero. The inductance is varied automatically to maintain theratio of voltage to current in the transmission line at a value equal tothe charach teristic impedance of the transmission line. Thus, the loadimpedance is matched to the impedance of the transmission line.

For a more complete understanding of our invention, reference should behad to the accompanying drawing. Fig. l of this drawing is a simpleschematic diagram or one embodiment of l our invention, which isdescribed in detail below,

.transmission line by coupling inductor line 8.

while Figs. 2a and 2b are an electrical `circuit diagram of thisembodiment of the invention. Figs. 3 and 4 are schematic diagrams of two-modications of our invention.

Referring to Fig. 1 of the drawing, the materia l to be heated is showndiagrammatically between heating electrodes 2 and 3. In series with theheating electrodes'isvariable capacitor which is operated by reversiblemotor 5. ln parallel with the heating electrodes is variable inductor 6operated by reversible motor i. rlhe net-work composed of electrodes 2and 3, variable capacitor 4 and variable inductor 6 is supplied withhigh frequency energy by generator 43 throughy transmission line t.Generator 43 is coupled to the InM ductor 9 is also used in conjunctionwith variable capacitor lil to form the oscillatory or ank circuit ofthe generator.

The impedance ofthe load network is automatically matched to theimpedance of the transW mission line by a phase control unit it and anamplitude 'comparison control unit irl. l'ioth control units receive asignal or control voltage from probe il through line I2 which isproportional to and in phase with the voltage in transmission Likewise,yboth control units receive a control voltage from loop Itr through lineiii which is proportional toand separated by ninety electrical degreesfrom the current in transmission line 3.l Control units lily and itfunction to adjust capacitor 4 and inductor t automatically to maintaina balance between the load iinped ance and the impedance of thetransmission line. Control unit i3 controls variable capacitor '4through reversible driving motor 5 and operating relays il and I8.Control kunit i4 controls varie able'inductor 6 through reversibledriving motor 1 and operating relays It and 2S.

Referring to Figs. 2a and 2b of the accom panying drawing, whichillustrate this embodH ment of our invention in detail, the materialflto 'be heated is shown in Fig. 2a supported. by conveyor belt la betweenheating electrodes 2 and 3, each of "which may be approximately 5:square Vfeet in area. The two electrodes are uration.

parallel and may be rectangular in conn The spacing of electrodesZ and 3is vari .aie arange several inches to accommodate variou heating loads.ff

f `Variable capacitor 4, in series with the heating electrodes'employsair as a dielectric and is coinm posed of three Vertical parallelplates. n The'outer 'two plates comprise one pole and the center platethe other pole, with the outer plates being i variable with respect tothe center plate by means of a worm gear driven by reversible motora.Additional details on the construction and operation of inductor B andits driving motor 'I are given below.

In one typical apparatus embodying our invention, generator 43 is of theoscillatory electron discharge type arranged to oscillate at a frequencyof 13,560 kilocycles. The generator is a conventional high frequencyoscillator comprising two valves 43a and 43h which have their controlelectrodes connected together with the two valves connected to operatein phase opposition in conjunction with an oscillatory circuitcomprising inductor member 9b and variable capacitor I9. Transmissionline 8 which connects the high frequency generator to the load is of theconcentric conductor type utilizing air as the dielectric.

Coupling inductor 9 is of the coupled hairpin type, that is, itcomprises a fixed member 9b and a movable member 9a both shapedapproximately like large hairpins with the two ends on both membersprojecting downward. Coupling between the two members is increased bymoving the movable member 9a closer to stationary member 9b whilemaintaining the planes of the two members parallel. Variable capacitorwhich is composed of two capacitor units in series, is of thepressurized gas-filled type with dry nitrogen gas as the dielectric.

In transmission line 8 is inserted probe II, which acts as a potentialdivider between the center conductor and outer conductor of transmissionline 8 to derive a small signal voltage that is proportional to and inphase with the transmission line voltage at that point. The signalvoltage derived by probe I I is transmitted through a small concentricconductor cable I2 to the phase sensitive control circuit and theamplitude comparison control circuit. Also inserted in transmission line3 is loop I5, which through its linkage with the electromagnetic fieldwithin line 8 has induced therein a small signal voltage whose magnitudeis proportional to the current in the transmission line and the phase ofwhich is 90 degrees behind that of the current in the transmission line.The signal voltage from loop adjust the capacitance of variablecapacitor 4 so that the phase angle in transmission line 8 is zero. Theamplitude comparison circuit is responsive to the ratio of voltage tocurrent in transmission line 8 and it adjusts the inductance of variableinductor through the action of reversible motor I actuated by a relaycircuit including relays I9 and 20 so that the ratio of voltage tocurrent is equal to the characteristic impedance of transmission line 8.

In a typical apparatus embodying our invention, operation is begun byenergizing the control circuits (not shown) of the high frequencygenerator 43. Thereafter, energizing potential is applied to the anodesof the high frequency generator to energize transmission line 8 and theheater network; simultaneously switches 2I and 22 are operated toenergize lines 23 and 24. En-

ergizing lines 23 and 24 energize the control circuits associated withmotors and I and motors 25 and 26.

Reversible motor 25 operates variable capacitor I0 in the tuned circuitof high frequency generator 43 through gear speed reducer 25a. Motor 25,in turn, is under the control of an automatic frequencyregulator (notshown) which is responsive to the frequency of the generator. Theautomatic frequency regulator operates switches 2T and 28 to operatemotor 25 in the required direction to secure the proper variation incapacitor I0 to maintain the frequency within desired limits which maybe, for example, 13,560 kilocycles plus or minus .05%. When switch 21 isclosed by the automatic frequency regulator, solenoid 29 is energized,closing switches 30 and 3|. This energizes windings 32 and 33 of motor25, and the motor rotates in the forward direction. When switch 28 isclosed by the regulator, solenoid 34 is energized, closing switches 35and 36. This energizes windings 32 and 31 of motor 25, and the motoroperates in the reverse direction.

Coupling inductor 9 in the tank circuit of the high frequency generatoris operated by motor 26 through a gear speed reducer 25a and a scissorsjack mechanism 2Gb. When the anode circuit of the high frequencygenerator is closed and switch 22 is moved simultaneously to the upperposition, solenoid 38 is energized, closing switches 39, 40 and 4I. Thisenergizes the windings of motor 26 so that the motor operates in theforward direction and moves movable member 9a of inductor 9 slowlytoward the maximum coupling position. When movable member 9a reaches apredetermined position, limit switch 42 operates, opening the circuit tosolenoid 38 and deenergizing motor 26.

When starting operation of the heater, motor 26 operates at slow speedto move inductor member 9a to the position which produces the desiredamount of coupling, and during this time the automatic tuning systemoperates to match the load and transmission line impedances. Thus, onlysmall amounts of power and relatively low voltages eXist in thetransmission line and heater circuits during the initial mismatchedcondition.

In the operation'of the automatic tuning system, the phase sensitivecircuit receives a signal voltage from probe II through concentricconductor I2 which appears on control electrode 44a of electrondischarge device 44. Concentric conductor I2 is terminated in a resistor45, the impedance of which is equal to the characteristic impedance ofconductor I2 to minimize power reflections and standing waves onconductor I2.

Electron discharge device is a oca-.n poi-'1er tetrode connected into aconventional crystal os cillator circuit. The piezoelectric crystal allmay have a fundamental frequency of. for ere 4,020 kilocycles, and theanode circuit of device 4G comprising inductor 4S and capacitor 49 istuned to this frequency. A resonant circuit comprising inductance 50 andcapacitor 5I is tuned to the third harmonic frequency, i. e. 12,060kilocycles, of crystal 4l and the voltage of this harmonic frequencyacross the resonant circuit 5I is impressed on a control electrode do ofelectron discharge device 14 and control eectrode 52h of electrondischarge device 52.

Electron discharge device 44 is a conventional type of pentagridconverter or mixer. With a 12,050 kilocycle signal applied to controlelectrode 4419 and a 13,560 kilocycle signal applied to controlelectrode 44a., the output current of this tube contains componentscomprising both the 4sum and difference of the two input frequencies.

tron discharge device 55.

standing waves thereon.

Theanode circuit of electron discharge device 44 comprising capacitor 53and inductance 54 is ktuned to the difference frequency of 13,560 minus12,060 or 1,500 kilocycles. High frequency components are by-passed toground through capacitor 55. On large signals, the output voltage ofelectron discharge device 44 increases only a fraction when theinput'signal voltage applied to electrode 44a increases several times.Hence, electron discharge device 44 functions also as a limiter on largeinput signals.

The 1,500 kilocycle voltage developed across resonant circuit 53, 54 isimpressed on control electrode 5ta of electron discharge device ithrough capacitor 5l. Electron discharge device 55 functions as alimiter, that is, the outputv voltt f age of device 5t is substantiallyconstant regardless of the magnitude of the signal voltage applied tocontrol electrode 56a. This limiting action is obtained by the use of aresistor 58 to provide a grid bias which increases with increasing.signal input and the use of a low screen voltage rto reduce the anodecurrent. The low screen voltage is obtained by the use of largeresistors 55 and 60 in the screen electrode circuit of elec- Capacitor6I and inductance 62 constitute a resonant circuitl tuned to a frequencyoi 1,500 kilocycles in the output circuit of electron discharge device55. A resistor 63 provides this circuit with a relativelybroad band passcharacteristic. Due to the amplitude limiting action of device 56, theoutput voltage of f tuned circuit 6I, t2 remains substantially constanteven though the signal voltage from probe II varies over a wide range ofamplitudes.

The constant voltage output of device 56 is impressed on controlelectrode 64a of an electron discharge device 64. Device operates as a'phase detector, and its operation is described in detail in the patentapplication Serial No. 62,172v of J .Vahle, F. E. Goodness and P. D.Heath, filed concurrently herewith and which is assigned to the assigneeof the present application.

The signal voltage induced in loop i5 is transmitted through concentricconductor it and appears on control electrode 52a of electron dischargedevice 52. Concentric conductor i8 is terminated in resistor theimpedance of which is equal to the characteristic impedance ofyconductor I6 to minimize power reiiections and Concentric conductor I6and concentric conductor I2 are of equal length so that equal phasedelay occurs in both conductors.

When the 13,560 kilocycle signal voltage from loop I5 is impressed onelectrode 52a and the 12,060 kilocycle voltage from the resonant circuit50, 5| is impressed on electrode 52h, electron discharge device 52functions as a mixer and limiter in a manner similar to that of electrondischarge device 44. rIhe frequency of the output current of device 52is 1,500 kilocycles, the same as device 44. The output voltage of device52 is impressed upon the input electrode of electron discharge devicee6, which functions in a manner similarto that of device 55 to provide aconstant amplitude output voltage regardless of variations in the signalvoltage from loop I5. This constant output voltage is impressed oncontrol electrode 54D of electron discharge device 54.

The anode circuit of electron discharge device 64 is supplied with aunidirectional operating voltage from a conventional voltage regulatedpower source (il. The output voltage of device 64 may vary, for example,from +30 volts'D. C,

to +200 volts D. C. when operated from a 250 volt D. C. supply, thisvoltage variation corresponding to a variation in phase angle betweenthe current and voltage in transmision line 8. of minus 90 degrees toplus 90 degrees. The output voltage of device 84 is impressed on thecontrol electrode of electron discharge device 68 through a voltagedropping resistor 69. The cathode of device 68 is maintained at aconstant positive potential with respect to ground by an electrondischarge device I0 of the gas filled voltage regulator type. Hence, theunidirectional output voltage of device 64 varies between predeterminednegative and positive voltage limits relative to the cathode of device68. This voltage is impressed on the grid of device 88 except that voltiable rheostats 'II and IZ connected in parallel respectively with relaysI'I and I 8 provide means to adjust the relative currents in the twosolenoid relays so that both of a pair of switches T3 and "i4,controlled by the two solenoids, are open when the automatic tuningsystem is in balance.

Switch I3 operates inversely to switch 14, the former being of thenormally closed type while switch I4 is ofthe normally open type. Whenthe system is completely deenergized, switch I3 is closed and switch 'I4is open. As the current flowing through the series circuit composed ofsolenoids I1 and I8 increases, solenoid I'I operates, at a predeterminedminimum amount of current, to open switch 13. With a continued increasein current, solenoid I8 operates at a second and higher predeterminedminimum amount of current to close switch 14.

When additional capacitance is required to adjust the phase anglebetween current and voltage in transmission line 8 to zero, solenoid Ilis deenergized suiiciently that switch 'I3 closes. Thisenergizessolenoid l5, closing switch `:16. This, in turn, energizessolenoid 'I'I which closes three phase contacter 18. The closingofcontactor 'I8 operates motor 5 in the forward direction through gearspeed reducer 5a to increase the capacitance of variable capacitor 4surnciently to return the phase angle in transmission line 8 to zero.

When a decrease in capacitance Ais necessary to restore the phase anglein transmission line 8 to Zero, solenoid I8 operates to close switch 14.This energizes solenoid I9 which closes switch 88. This in turnenergizes solenoid 8l which closes three phase contactor83. Closingcontacter 83 operates motor 5 in the reverse direction until thecapacitance of variable capacitor 4 is reduced sufficiently to restorethe phase angle in transmission line 8 to zero.

Variable capacitor 4 is operated at a rapid rate compared to variableinductor 6, the operation of which is described below. In one typicalapparatus embodying our invention capacitor 4 is variabie from theminimum position of 30 micromicrofarads to the maximum position of 200micromicrofarads in 5 seconds. The tuning system is adjusted so that theswitch 'I3 closes to restore the phase angle to zero if the phase anglein transmission line ii becomes less than -5, whereas switch 'F4 closesif the phase angle exceeds +5. y

In the operation of the amplitude comparison control circuit a signalvoltage from probe II is transmitted through coaxial conductor I i. and

- 'afee 'che 1 -7 impressed on one anode of a vduo-"diode electrondischarge device 84. Concentric conductor I2 is terminated in resistor85, the impedance'cf which is equal to the characteristic impedance ofcon- -ductor I2 to prevent power reections and standing waves onconductor I2. The signal voltage 'from loop I5 is transmitted throughconcentric conductor i6 and is impressed on the cathode of difference inthe amplitudes of the signal voltages n from probe |I and loop I5. Theoperation or" device 84 is described in detail in the application S. N.62,174 of Paul D. Heath filed concurrently herewith and which isassigned to the assignee of the present application.

The output voltage of device 84 is impressed on the input electrode ofan electron discharge device 88 which functions as a conventionalamplifier. The output voltage of device 88 'is in turn impressed on thecontrol electrode of an electron discharge device 89 through a voltagevdropping resistor 82. Device 89 functions in a manner similar to that ofdevice 64. The voltage from device 88, which is impressed on the grid ofdevice 89 varies between predetermined positive unidirectional voltagelimits. The cathode of device 89 is maintained at a xed positivepotential above ground potential by an electron discharge device 900ithe gas filled voltage regulator type. Thus, the output voltage ordevice 88 varies within predetermined negative and positiveunidirectional voltage limits relative to the cathode of device 89. Thisvoltage is impressed on the control electrode of device 89 except thatvoltage dropping'resistor 82 Yprevents the control electrode potentialfrom becoming positive.

The anode circuit of device 89 is energized by connection with aconventional voltage regulated` power source 9|. 89 is used to operatesolenoid 'relays I9 and 20. Variable rheostats 92 and 93, connectedrespectively in parallel with solenoid relays I9 'and 20, provideadjustment of the relative currents in the two solenoid relays so thatboth of the switchesy 94 and 95, which are operated by theAtwosolenoids, are 'open when the 'system is 'in balance. Switch 94 isof the normally closed type while switch 95 is of the normally opentype. Switch 94 remains closed unless the current Iin solenoid. I9exceeds a predetermined amount while switch 95 remains open unless thecurrent in solenoid 20 exceeds a second higher predetermined amount.

When the signal voltage from transmission line 8 indicates thatadditional inductance is required to match the load impedance to thetransmission line impedance, solenoid I9 operates to close switch 94.This energizessolenoid 96 to 'close switch 97. This in turn energizessolenoid98 to close three phase contactor 99. The closing of contactor99 operates motor 'I in the forward'direction through gear speed reducer1a to increasethe spacing between the two members of variable inductor 6and thus increase its inductance sufli- The output voltage of device ssd8' ciently to raise the total load impedance to the desired amount.

When the signal voltage from transmission line 8 indicates that lessinductance is required, solenoid 20 operates to close switch 95. Thisenergizes solenoid |00 to close switch IDI. This in turn energizessolenoid |02 to close three phase contactor |03. Closing contactor |03operates motor 'I in the reverse direction to reduce the spacing betweenthe elements of variable inductor 6 thus reducing the inductance ofinductor 6. Consequently, the total load impedance is reduced. Motor 'Icontinues operation until the load impedance is equal to the impedanceof transmission line 8.

A conductor |04 connecting switch IOI to switch 22 is provided to returnvariable inductor 6 to its lowest or minimum inductance position whenoperation of the automatic tuning system is stopped. When the anodecircuit ofthe high frequency generator is opened to stop operation,switch 22 returns to the lower position to ener- ;gize solenoid |02 andclose three phase contactor |03. This contactor operates motor 'I in thereverse direction in the same manner as it is operated under the controlof the amplitudev comparison circuit. Motor l operates to lower theupper member of variable inductor 6 until limit 4switch |65 is operatedto open the circut to solenoid |02, deenergize the control circuits tomotor 1, and stop the movement of variable inductor 6.

Tuning inductor 6 operates at a relatively slow rate compared tovariable capacitor 4. Inductor 6 may travel the full range from minimuminrductance position to the maximum inductance po- :sition inapproximately 30 seconds. During the time that inductor 6 is in motionin either direction variable capacitor 4 is continually adjusted by thephase sensitive control circuit to restore the phase angle intransmission line 8 to Zero. nSince capacitor 4 operates at aconsiderably faster rate than inductor E, it is possible for the formerto complete several operations during one operation of the latter.

Variable inductor 6 is composed of parallel at plates with a flexibleelectrically conductive connection at one end to form a short-circuitedsection of transmission line. The spacing between the plates isadjustable to vary the inductance. Inductor 6 is described in detail inapplication S. N. 62,171 of Julius Vahle, now Patent No. 2,494,596,filed concurrently herewith and which is assigned to the assignee of thepresent application.

Coupling inductor 9 in the tank circuit of the :high frequency generatoris connected so that it also returns to the minimum coupling and theminimum reactance position whenever the main heating circuits of theapparatus are deenergized by opening the anode circuit of the highfrequency generator. When the anode circuit ofthe generator is opened,switch 22 is returned to the lower position indicated on theaccompanying drawing. This energizes solenoid |01 which moves switch |68to the upper position and closes switches |09 and III). This energizesthe'windings of motor 26 in a manner such that the motor operates in thereverse direction and movesthe .movable member'ila of inductor 9 towardthe .minimum coupling and minimum reactance position. This motioncontinues until limit switch IIIY opens, deenergizing solenoid |01 andstopping motor 26.

An important feature of our invention isan interlock onl motor 25driving variable capacitor IB which prevents the operation of the otherthree motors associated with the automatic tuning system during the timethat motor 25 is operating. By operating motor 25 only when thefrequency of the generator lies outside the selected frequency bandlimits, the automatic tuning system does not function during the timethat the frequency of the generator does not fall within these limits.This control is provided by switches H2 and IIS, one of which is openedwhen either of solenoids 29 or 34, which control the operation oi motor25, is energized. If either of switches II2 or H3 is opened, all controlcircuits for motors 5, l and 26 operating respectively variablecapacitor 4, variable tuning inductor E and coupling inductor 9 aredeenergized. This assures that these motors will stop if they arerunning when either switch H2 or switch II3 is opened and they cannotstart again until motor 25 has ceased to operate and both of switchesIIZ and I I3 are closed. This interlocking eliminates spurious responseof the automatic tuningl system when the frequency is outside the bandlimits, and reduces unnecessary interaction between the various elementsof the automatic tuning system` Fixed capacitors I I4 may be provided inseries with the load electrodes 2 and 3 to reduce the large capacitanceof the load electrodes and make it possible to use a smaller variableinductor '6 in parallel therewith for tuning purposes.

ln Fig. 3 there is illustrated one modification of our invention. Thishas a Variable inductance 4 substituted for variable capacitor 4 in theautomatic tuning system and is suitable for use with loads requiringseries inductance rather than capacitance to match the load impedance tothe transmission line impedance. The two variable inductances in thismodification may be any conventional type such as the variable stub typeor the rotating lcoil type, or they may be of the type illustrated byinductor of Figs. 1 and 2.

Another modiiication is illustrated by Fig. 4 in which there is provideddifferent means for obtaining the signal voltages for the amplitudecomparison control circuit. In this form of our invention two probes areinserted in the concentric conductor transmission line spaced apart by adistance of from r1/10 to A wavelength. The signal voltages from thesetwo probes are proportional to the standing wave ratio in thetransmission line which is a measure of the power reiiected back fromthe load due to impedance mismatch. The two signal voltages are used tocontrol the amplitude comparison circuit so that the standing wave ratiois adjusted to a minimum. at which point the load impedance andtransmission line impedance are equal. In other words, the variableinductor in parallel with the load is adjusted so that the signalvoltages from the two probes are equal.

While we have illustrated and described one preferred embodiment of ourinvention, together with two modications, many additional modicationswill occur to those skilled in the lart. It should be understood,therefore, that we intend to cover by the appended claims any suchmodirications as fall within the true spirit and scope of our invention.

What we claim as new and desire to secure by Letters iatent of theUnited States is:

l. A tuning system for a load supplied with high frequency energy b v atransmission line, comprising means for deriving a signal proportionalto and in phase with the voltage in said transmission line, means forderiving a signal proportional to and out of phase with the current insaid transmission line, a variable reactance in series with said load,means utilizing said two signals for deriving the phase angle betweenthe current and voltage in said transmission line, means responsivetosaid phase angle deriving means for varying said series reactance adjustsaid phase angle to a preselected value. a variable reactance inparallel with said load, means for determining the ratio of transmissionline impedance to load impedance, and means responsive to ratiodetermining means and operative simultaneously with said seriesreactance varying meansrfor varying said parailel reactance to adjustsaid ratio to a predetermined value.

2; A tuning system for a load supplied with high frequency electricalenergy by a transmis-- sion line, comprising means for deriving a signalproportional to and in phase with the voltage in said transmission line,means for deriving a signal proportional to and out of phase with thecurrent in said transmission line, a variable capacitive reactanceconnected in series circuit relationship with said load, means utilisingsaid two signals for deriving the phase angle' between the current andvoltage in said transmission line, means responsive to said phase anglederiving means for varying said seriescapacitive reactance to adjustsaid phase angle to substantially zero, a variable inductive reactanceconnected in parallel with said load, means utilizing said two signalsfor deriving the ratio of voltage to current in said transmission line,and means responsive to said ratio deriving means and operativesimultaneously with said series reactance varying means for varying saidinductive reactance to adjust said ratio to a predetermined value.

3. An automatic tuning system for a high frequency heater, the load ofwhich is provided with electrical energyby means of a transmission line,comprising means for deriving a signal voltage proportional to and inphase with the voltage in said transmission line, means for deriving asignal'voltage proportional to and out oi phase with the current in saidtransmission line, a variable capacitive reactance in series with saidload, means utilizing said two signal voltages for deriving the phaseangle between the current and voltage in said transmission line, meansresponsive to said phase angle deriving means for varying said seriescapacitive reactance to adjust said phase angle to a preselected value,a variable inductive reactance in parallel with said load, meansutilizing said two signal voltages for determining the ratio `oi?transmission line impedance to load impedance, and means responsive tosaid ratio determining means and operative simultaneously with saidcapacitive reactance varying means for varying said inductive reactanceto adjust said ratio to a predetermined value.

4. An automatic tuning system for a high frequency heater, the load ofwhich is provided with heating energy by means of a concentric conductortransmission line, comprising probe means for deriving a signal voltageproportional to and in phase with the voltage in said transmission line,loop means for deriving a signal voltage proportional to and separatedby ninety electrical degrees from the current in said transmission line,a variable capacitive reactance in series with said load, meansutilizing the two said signal voltages for determining the phase anglebetween the current and voltage in said transmission line, meansresponsive to said phase angle 11` determining means for varying saidseries capacitive reactance to adjust said phase angle to Zero, avariable inductive reactance in parallel with said load, meansutilizing'the.. two said signal voltages for determining the ratio ofvoltage to current in said transmission line, and means responsive tosaid ratio determining means and.

operative simultaneously with said series reactance varying means forvarying said parallel inductive reactance to adjust said ratio to avalue equal to the characteristic impedance of said transmission line.

5. An automatic tuning system for a high frequency electrical heatingapparatus, the load of which is provided with heating energy by means ofa transmission line, comprisingprobe means for deriving a signal voltageproportional to and in phase with the voltage in said transmission line,loop means for deriving a signal voltage proportional to and separatedby ninety'electrical. degrees from the current in said transmissionvline, a variable capacitive reactance in series' with said load,reversiblemotor means for operating said capacitive reactance, meansutilizing said two signal voltages for deriving a voltage responsive tothe phasefangle between vcurrent and voltage in said transmission line,relay means responsive to said derived voltageifor operating said motormeans, whereby said capacitive reactance is adjusted to maintain saidphase angle substantially zero, a variable inductive'reactance inparallel with said load, reversible motor means for operating ,saidinductivereactance, meansv queries.` heating apparatus, the load ofwhich is` .ed with electrical heating 'energy by means of a concentricconductor transmission line, for

matching the impedance of the'load and the'impedance ot the transmissionline, comprising a generator width, means for coupling said generator tosaid transmi .ion line, a probe positioned in said transmission linebetweenV the two conductors for deriving a signal voltage proportionalto and in phase with the voltage in the transmission line, a looppositioned in said transmissionline between the two conductors forderiving a signal voltage proportionalto and separated by ninetyelectrical degrees from the current in the transmission line, a variablecapacitive reactance connecte in series with said load, reversible motormeans for operating said capacitive reactance, means utilizing said twosignal voltages for deriving a voltage responsive to the phase anglebetween current and voltage in said transion line, relay meansresponsive to said deed voltage for operating said motor means, wherebysaid capacitive reactance is adjusted to maintain said phase anglesubstantially zero, a variable inductive reactance in parallel with saidload, reversible motor means operative simultaneously with saidcapacitive reactance motor means for operating said inductive reactance,means utilizingsaid two signal voltages for deriving a voltageresponsive to the ratio of the transmission line impedance to the loadimhaving a selected frequency band.

pedance, relay means responsive to: said.. lastnamed. derived voltage`and operative .simule taneously with saidifirst-'namedrelay means foroperating said last-named. motor means, whereby said.inductivereactanceis adjusted to. maintain said ratio at a .predetermined value, and

means .associated with. said generator for dis-- continuing operation ofsaid capacitive reactance. motor means and said; inductive reactancemotor;

means when 'the frequency of saidy generator is outsidefthe selectedband limits.

'7. Antautomatic tuning system for a highA frequency heater, the load ofwhich is provided withv heatingenergyV by Ameans of .a concentric con- 1ductor transmission line, comprising probe-means` for deriving a signalvoltage proportional to and inphasevwith the voltage in saidtransmission line, loop means for'deriving a signal voltageAproportional to and separatedby ninety electrical degreesfrom the.currentin-.said trans.

mission line, a` variable capacitive reactance in series with saidVload, means utilizing the two saidisignal voltages forV determining thephaseV angle betweenthe current'and voltage in said transmission line;means responsive to said phase angle determining meansffor varying saidseries capacitive reactanceftoadjustrsaid phase angleto zero, a variableinductive reactancein parallel with said load, double probe-means forderiving a signal voltageA `proportional to the standing waveratioinsaid transmissioniline, and means responsive to .saidlast-named signalvoltage and operative simultaneously* with; said series reactancevarying means forvarying said parallel j., inductive reactanceautomaticallyV to` maintainy said standing Vwave ratio :at unity.

8. An automatic tuning system for aY high frequencyY heater, `the load'of` Whiclris provided with electrical energy by means of a transmission.

line, comprising means vfor deriving a signal voltage-proportional toandinY phaseV withL the voltagen in said transmission line, means forderiving-a signal voltageV proportional to and out ofphase withthecurrent in said transmission line,V a variable inductive reactance inseries with said load, means utilizing said two signal voltages fordetermining the -phase angle between theA current and voltage insaidtransmission line,` means responsive to said phase angle determiningmeans forYvarying/series reactance to adjust said phase anglestoapproximately zero, a variable inductive reactance in parallel with saidload, means utilizing said two signal voltages for determining the ratioof voltageto current in saidvtransmission line, and means responsive tosaid ratio, determining means and operative simultaneously with saidseries reactance varying means forvarying said parallel reactance toadjust said ratio to a predetermined value.

JULIUS VAHLE. PAUL D. HEATH.

REFERENCES CITED The following references are of record in the.

nie. of this patent:

UNITED STATES PATENTS Numberv Name Date 2,438,595 Zottu Mar. 30, 19482,456,800 Taylor Dec. 21, 1948 OTHER REFERENCES Radio Engineering byTerman (3rd edition) published in 1947 by McGraw Hill.

